Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and related techniques are commonly referred to simply as MIMO.
The 3GPP LTE standard is currently evolving with enhanced MIMO support. A core component of this support in LTE is the support of MIMO antenna deployments and MIMO related techniques. A current working assumption in LTE-Advanced is the support of an 8-layer spatial multiplexing mode for 8 transmit (Tx) antennas, with the possibility of channel dependent precoding. The spatial multiplexing mode provides high data rates under favorable channel conditions.
With spatial multiplexing, an information carrying symbol vector s is multiplied by an NT×r precoder matrix QNT×r, which serves to distribute the transmit energy in a subspace of the NT (corresponding to NT antenna ports) dimensional vector space. The precoder is typically selected from a codebook of possible precoders, and typically indicated by means of a precoder matrix indicator (PMI). The PMI value specifies a unique precoder in the codebook for a given number of symbol streams.
However, certain challenges arise in this context. For example, different antenna configurations can require precoder structures of one type or another, which complicates the storage of predefined codebooks of precoders. Still further, the dynamic use of Single User (SU) MIMO and Multi-User (MU) MIMO modes complicates codebook design because precoders that are optimal for SU-MIMO generally will not be optimal for MU-MIMO. As a further complication, the overhead associated with reporting precoder information, e.g., precoder recommendations, from a receiver to a transmitter may be problematic. This is true, for example, in the LTE downlink where the Physical Uplink Control Channel (PUCCH) cannot bear as large a payload size as the Physical Uplink Shared Channel (PUSCH).